48 BIOLOGICAL SOCIETY OF WASHINGTON. 



ble to their preservation, as compared with those of aqueous ani- 

 mals, which have had ahnost immediate sepulture. 



I fear that in consequence of these remarks, following those with 

 which I began to address you, some will be ready to suggest that 

 naturalists have insecure ground to base their generalizations upon, 

 as regards the origin of the existing animals of the earth by lineal 

 descent from extinct faunas. I fully recognize the danger, in such 

 an address as this, of giving undue prominence to the doubtful side 

 of the subject. It is difficult also to satisfactorily present the af- 

 firiTiative side in a brief and concise manner, because that side is 

 supported by evidence which is cumulative in its character, rather 

 than reducible to precise propositions. 



My object, however, in showing how completely great faunas 

 may have been destroyed in past geological time is to plausibly ac- 

 count for the absence of their remains in places where our methods 

 of reasoning lead us to expect them ; and also to show that, because 

 their remains have never been discovered, we should not necessarily 

 infer that the animals which were necessary to complete a regular 

 genetic scale never existed. Indeed, the fact that certain breaks in 

 the zoological scale occur at certain horizons of the geological scale 

 ought to lead us to infer that the missing animal forms did exist 

 somewhere at such times, rather than that they never existed at all. 



Now, as the study of the genetic descent of animals through geo- 

 logical time is based upon plan of structure, and the methods by 

 which form is expressed, these indications may be ranged under two 

 heads, namely, similarity of structure and identity of type. The 

 former is a matter of tangible details, but the latter is in some sense 

 ideal, or a manner in which form, in connection with structure, is 

 expressed. The former is material in its character, but the latter is 

 not the less real and important to the naturalist in the philosophi- 

 cal study of the comprehensive groups of animals. 



The word "type," like many other words in the English lan- 

 guage, is used with a variety of meanings; and as I use it here 

 in a special sense, I may be excused for adding the following 



